1. Field of the Invention
The present invention relates to a pattern inspection apparatus and a pattern inspection method. For example, it relates to an apparatus and method for inspecting patterns by using pulsed light and a time delay integration (TDI) sensor.
2. Description of Related Art
In recent years, with high integration and large capacity of large scale integrated (LSI) circuits, the line width (critical dimension) required for circuits of a semiconductor element is becoming narrower and narrower. The semiconductor element is manufactured by exposing (transferring) a pattern onto a wafer to form a circuit by a reduced projection exposure apparatus, known as a stepper, while using an original or “master” pattern with a circuit pattern formed thereon. (The original pattern is also called a mask or a reticle, and hereinafter generically referred to as a mask). Therefore, in manufacturing a mask for transferring such a fine circuit pattern onto a wafer, a pattern writing apparatus using electron beams capable of writing or “drawing” fine circuit patterns needs to be employed. Pattern circuits may be written directly onto a wafer by the pattern writing apparatus. In addition to the writing apparatus using electron beams, a laser beam writing apparatus which uses laser beams for writing patterns is also under development.
Since the LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as represented by a 1 gigabit DRAM (Dynamic Random Access Memory), the order of a pattern constituting an LSI has been changing from submicron to nanometer dimensions. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when exposing (transferring) a fine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions of defects to be detected have become extremely small. Thus, a pattern inspection apparatus for inspecting defects of a mask for exposure used in manufacturing LSI needs to be highly accurate.
Meanwhile, with development of multimedia technology, the size of Liquid Crystal Display (LCD) substrates is becoming larger, e.g., 500 mm×600 mm or greater, and the size of a pattern such as a Thin Film Transistor (TFT) or the like formed on the liquid crystal substrate is becoming finer. Therefore, it is increasingly required that an extremely small defect of a pattern should be inspected in a large range. For this reason, development of a pattern inspection apparatus which efficiently and short-timely inspects defects of a pattern of a large area LCD and a photomask used in manufacturing the large area LCD is urgently required.
As an inspection method, it is known that an optical image of a pattern formed on a target object or “sample”, such as a lithography mask, imaged at a predetermined magnification using a magnifying optical system is compared with design data or an optical image of an identical pattern on the target object. For example, the following is known as pattern inspection methods: “die to die inspection” method that compares data of optical images of identical patterns at different positions on the same mask, and “die to database inspection” method that inputs into the inspection apparatus the writing data (design pattern data) converted from pattern-designed CAD data to a format for input to the writing apparatus when writing a pattern on a mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (measurement data) obtained by capturing an image of the pattern. According to the inspection method of the inspection apparatus, a target object is placed on a stage so that a light flux may scan the object by the movement of the stage. Specifically, the target object is irradiated with a light flux from the light source and the illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to a comparison circuit. In the comparison circuit, after position alignment of the images, measurement data and reference data are compared in accordance with an appropriate algorithm. If there is no matching between the data, it is judged that a pattern defect exists.
Now, when inspecting a mask with a pattern formed thereon, a light of short wavelength (ultraviolet band) needs to be used as an illumination light in order to inspect an extremely small detect by the inspection apparatus. Conventionally, as an illumination light, there has been used a continuous wave light source which is controlled to have a fixed exposed light by feedbacking the light power. As a sensor for capturing a pattern image on the mask, there has been used a time delay integration (TDI) sensor which time delay integrates pixel values acquired by shifting a two-dimensional area (xy direction) sensor in x direction while being synchronized with the stage. Thus, by using the TDI sensor, the structure is employed in which shortage of sensitivity of the sensor element is compensated by exposing continuous light accumulated for a predetermined number of times. In contrast, there is disclosed a technique in which since fluctuation of exposed light may occur in a continuous wave light source that does not feedback the light power, a light exposure sensor is placed at the light source side to correct fluctuation of output from the TDI sensor by using an exposed light power of irradiating continuous light (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2004-101438).
The light of short wavelength (ultraviolet band) is not limited to the continuous wave light source mentioned above. For example, an ArF excimer laser light can be cited as the light of short wavelength. An excimer laser light source is smaller and more efficient than the conventional continuous wave light source. Therefore, it is expected to load the excimer laser light source as a light source of the inspection apparatus. However, since the excimer laser light is a pulsed light source, it produces luminescence of only a nanosecond (nSec) during a millisecond (mSec). Then, there is a problem when using this pulsed light as an illumination light, under the influence that an exposed light per pulse varies, an accumulated light of the sensor may considerably change even if a plurality of pulses perform irradiation during the time of the TDI sensor being shifted by one pixel. With respect to the continuous light, even if the light power changes, since the irradiation is always performed during the time of the TDI sensor being shifted by one pixel, the accumulated sensor exposure light can be well averaged. On the other hand, with respect to the pulsed light, since the irradiation time of during the shift by one pixel is much shorter than that of continuous light, the exposed light fluctuation of illumination light gives a great influence on it.
As mentioned above, there is a problem that when a pulsed light is used as an illumination light of the inspection apparatus, under the influence that the light power per pulse varies, the accumulated light to the TDI sensor may change larger than the case of using a continuous light.